An off-line power converter can be implemented using an integrated circuit power factor correction (PFC) controller. The PFC controller helps to boost the efficiency of power transfer to the load by making the current and voltage waveforms in phase with each other. To reduce electromagnetic interference (EMI), a typical offline converter with a PFC controller operates in critical conduction mode (CrM) in which a new switching cycle begins when the current through an inductor of the PFC stage drops to zero. The instantaneous inductor current varies from zero to a value that is proportional to the line voltage, and the average inductor current follows the same wave-shape as the input voltage, thus providing no distortion or phase shift in the current waveform.
In PFC controllers, it is difficult to preserve efficiency and to simultaneously achieve near-unity power factor across the entire load range. One solution to this problem is known as frequency-clamped CrM. As the level of the load lightens, a CrM controller with frequency clamped CrM clamps the natural switching frequency to preserve high efficiency. Once the frequency is clamped, the PFC controller enters discontinuous conduction mode (DCM). The power factor drops as the frequency varies from the natural frequency due to the frequency clamping if no circuitry to compensate for the dead-times is implemented. With frequency-clamped CrM controllers that incorporate circuitry to compensate for the dead-times, the power factor remains high but some noise can be generated due hesitations between valleys, and some current bumps can be observed particularly at transitions between CrM and DCM operation.
Other PFC controllers use techniques such as valley-synchronized frequency foldback (VSFF) and current controlled frequency foldback (CCFF) that reduce the switching frequency at light loads by forcing some dead-time without providing a firm frequency clamp. These PFC controllers modulate on-time to compensate for dead times experienced in DCM. They achieve high power factor at light loads but do not firmly control the frequency range. They may also experience current bumps when switching between valleys or transitioning between CrM and DCM.
The use of the same reference symbols in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.